EXCRETION BY THE KIDNEY. 425 
carbonates as is not derived directly from a corresponding sup- 
ply in the food may be traced to the oxidation of certain or- 
ganic salts, as citrates, tartrates, etc. 
Doubtless many bodies appear either regularly or occasion- 
ally in urine that have so far escaped detection, which are, like 
the poisonous exhalations of the lungs, not the less important 
because unknown to science. 
Abnormal Urine.—There is not a substance in the urine that 
does not vary under disease, while the possible additions act- 
ually known are legion. These may be derived either from 
the blood or from the kidneys and other parts of the urinary 
tract. The kidneys seem to take upon themselves more readily 
than any other organ the duty of eliminating foreign matters 
from the body. But this aspect of the subject is too wide for 
detailed consideration in this work. 
The student of medicine should be thoroughly familiar with 
the urine in its normal condition before he enters upon the 
examination of the variations produced by disease. This is 
not difficult, and much of it may be carried out with but a 
meager supply of apparatus. For this purpose, however, we 
recommend some work devoted to the chemical and micro- 
scopic study of the urine. 
It greatly assists to remember a few points in regard to 
solubilities. From a physiological point of view, the urine and 
its variations, as the result of changes in the organism, may be 
observed with advantage in one’s own person—e. g., the influ- 
ence of food and drink, temperature, emotions, etc. 
Comparative —The urine of most vertebrates is of higher spe- 
cific gravity than that of man. In fishes, reptiles, and birds, 
uric acid replaces urea, and is very abundant. In these animals 
most of this substance is white. The urine is passed with the 
feces. Among mammals the urine of the carnivora is strongly 
acid, perhaps owing in great part to the flesh on which they 
feed; and abounds in phosphates and, in some instances, sul- 
phates. The urine is so concentrated in some cases that we 
have known urea nitrate to crystallize out on the addition of 
nitric acid without requiring condensation. 
The urine of the herbivora is alkaline, and abounds in salts 
of calcium, especially carbonates. It is also of high specific 
gravity, and grows rapidly dark in color when passed, owing 
probably to the presence of the aromatic compounds referred 
to above, derived from the food. In certain groups of inverte- 
brates uric acid seems to be a normal excretion. 
